I. Field of the Invention
The present invention relates to polytetrafluoroethylene based coating compositions containing polyvinylidene fluoride and lithia.
II. Description of the Prior Art
It is known that one can apply to metal or other surfaces polytetrafluoroethylene coatings so as to make use of the anti-adhesive properties of this polymer. These coatings are particularly interesting in the field of culinary utensils. However, as a result of the nature of polytetrafluoroethylene, the coatings normally achieved with this resin exhibit very poor adhesion to the substrata on to which they are applied.
It is known that the aforementioned adhesion may be improved by including in the coating compositions certain additives which are sometimes known as bonding agents. Although numerous compositions containing bonding agents have been proposed so as to bring about improved adhesive coatings of polytetrafluoroethylene, only two types of composition seem to be generally accepted. These are polytetrafluorethylene compositions containing a mixture of chromic acid and phosphoric acid and polytetrafluoroethylene compositions containing lithium polysilicates.
The compositions containing a mixture of chromic acid and phosphoric acid are described in particular in U.S. Pat. No. 2,562,118. They are obtained from an aqueous dispersion of polytetrafluoroethylene, the above acid mixture, and possibly other additives such as emulsifiers and pigments. While these coating compositions exhibit good adhesive characteristics, particularly when applied upon steel and aluminum, they nevertheless possess two very serious drawbacks. The first is that these compositions, which are highly charged with mineral acids, are not very stable such that it is necessary to maintain the acid components separate from the rest of the composition, and to only admix it with the rest of the composition just prior to application. A second drawback of the chromic acid and phosphoric acid containing coating compositions is that these compositions contain toxic hexavalent chromium. Thus, in various countries, including France, their application as coatings upon culinary utensils is not permitted.
The aforementioned compositions containing lithium polysilicate are described in French Pat. No. 2,024,171. They consist essentially of an aqueous suspension of polytetrafluoroethylene and lithium polysilicate. While these compositions may be applied on to culinary utensils, the adhesion of these coatings depends to a large extent on the exact nature of the lithium polysilicate employed. Also, the lithium polysilicate has to be free from alkali metals other than lithium and must have a molar ratio of SiO.sub.2 /Li.sub.2 O which must be regulated within very narrow limits and its preparation in an aqueous medium requires a ripening phase which is difficult to control. Finally, the adhesion of this type of coating is very much dependent on the nature of the dispersion of polytetrafluoroethylene used in its manufacture. Thus, it is apparent that coating compositions containing lithium polysilicate are both laborious to prepare and exhibit adhesive properties. which often are unsatisfactory.
Also, as regards the present state of art vis-a-vis polytetrafluoroethylene based coating compositions, it is useful to note that the compositions mentioned above, whether they are those containing a mixture of chromic acid and phosphoric acid or those containing lithium polysilicate, may be applied either alone or as primers which favour adhesion thereupon. In the latter case, which is by far the most frequent in practice, after treating the surface to be coated with one of the compositions described above, one applies a conventional polytetrafluoroethylene based finishing lacquer. By conventional polytetrafluoroethylene based finishing lacquer one means compositions containing polytetrafluoroethylene and various other additives, but not containing bonding agents. These lacquers, which when used alone yield satisfactory coatings relative to their appearance and surface properties, nevertheless exhibit very poor adhesion characteristics upon the substratum.